In its broad aspects, the present invention is directed to secondary electrical energy storage systems of the aqueous type. A metal halogen hydrate electrical energy storage system of the type to which the present invention is applicable is fully described in U.S. Pat. No. 3,713,888, entitled "Halogen Hydrates", issued Jan. 30, 1973. This patent is owned by the same assignee as the present invention and details thereof beyond those herein described are incorporated in this application by reference. Metal halogen hydrate electrical energy storage systems or secondary storage batteries are conveniently categorized as being of the high energy density (H.E.D.) type because of their capacity to supply upwards of 50 watt hours of electric power per pound of weight. This high electrical energy capacity coupled with the compactness and low weight of such secondary storage batteries has rendered them particularly satisfactory for use as principal and auxiliary sources of electrical energy in either mobile (electric vehicles) or stationary (utility load leveling) power plant systems.
The present invention pertains primarily to zinc halogen battery systems, and more particularly to zinc chlorine battery systems, although it should be appreciated that the invention described herein may be equally applicable to other metal halogen battery systems. The chemical reactions which occur in a zinc chlorine hydrate battery are relatively straightforward. During charge, the electrolyte (a solution of zinc chloride in water) is flowed through the battery with the aid of a circulator. As electrical direct current is passed through the battery from an external source, zinc metal is electro-deposited on the negative electrode (typically relatively dense graphite) of the battery as a uniform, non-porous solid. Simultaneously, chlorine gas, generated at the positive electrode (typically porous graphite or ruthenia-catalyzed porous titanium) is carried away with the circulating electrolyte stream. Outside of the battery, the chlorine gas is admixed with water cooled to less than about 10.degree. C. and a pale yellow solid called chlorine hydrate is formed. This reaction is exothermic with the heat of formation of chlorine hydrate being about -18 kCal/mole. Accordingly, the store area, where the solid chlorine hydrate (Cl.sub.2.xH.sub.2 O) is retained separate from the battery, is chilled continuously during charge. During discharge, the aqueous zinc chloride electrolyte is circulated through the battery thereby carrying chlorine, which is slightly soluble in the electrolyte, to the chlorine electrode of the battery and permitting current to be withdrawn from the battery. To replace the chlorine in the electrolyte, the chlorine hydrate is heated in a controlled manner to release chlorine from the hydrate.
As noted above, one proposed use of the above-described zinc chloride battery is in what is called utility load leveling, in which case, a series of batteries would be used at an electrical power plant to store off-peak electric energy to be used during periods of peak demand. This alternative is especially attractive in view of the current increasing demand for electricity and the high cost and short supply of petroleum oil. In this load leveling application, several individual zinc halogen batteries or battery modules would be arranged in strings to provide the desired electrical output. For example, systems under consideration at the present time include a 4 MWh (megawatthour) system, which is based on the use of eighty battery modules; and a 100 MWh system, which is comprised of four 25 MWh units each consisting of four strings made of 120 series-connected battery modules.
At present, the control and operation of a single zinc chloride battery is fairly well developed with such considerations as individual battery cooling, electrical connections, electrical control, and the like having been investigated with several solutions having been proposed. But the control and operation of a series of batteries differs considerably from the operation of a single battery. For example, with regard to a single battery, it has been common to cool the battery hydrate store area using such materials as ice water, cold brine, or glycol solutions, with such materials as fluorocarbon refrigerant in a conventional evaporation cycle having been proposed. However, when batteries are connected in series, there is an electric potential between the batteries, and hence any cooling system used for the batteries has to be non-conducting or dielectric. In addition, as will be described in more detail hereinbelow, the battery electrolyte sump area also requires cooling. Thus conductive materials such as ice water or brine are unsatisfactory, although deionized water, which is a dielectric, has been proposed. But the use of deionized water as a coolant would require a totally separate means of cooling the deionized water, thus introducing considerable capital expense into the system for a separate refrigeration plant including such equipment as heat exchangers, additional valving, and the like. The same would be true for glycol solutions, which also have the drawback of being combustible. With regard to fluorocarbon refrigerants such as Freon, even though their use as a refrigerant for the hydrate store area of an individual battery has been suggested, such suggested use has been in terms of a conventional evaporation cycle (Freon liquid in and Freon vapor out), and even then has never actually been tested. More importantly, none of the other associated scale-up problems involving a series of batteries such as temperature control and other heat transfer factors have been considered and solved until the present invention.
Therefore, one of the objects of the present invention is to provide a method of cooling a series of batteries using a dielectric coolant such as fluorocarbon refrigerant or the like which not only functions as the coolant, but also is readily adaptable to a conventional refrigeration loop for regenerating the coolant.
Other objects, features and advantages of the present invention will become apparent from the subsequent description, and example, and the appended claims taken in conjunction with the accompanying drawings.